Liquid crystal display devices are becoming evermore widely used in the process control industry In process control applications, meters with liquid crystal displays (LCD) are often self-powered from the instrumentation loops used to transmit process parameters such as temperature, pressure and flow rates. The process parameters are commonly sensed by transducers such as flow transmitters which transmit current proportional to the sensed process parameter. The amount of current transmitted, typically 4-20 milliamps, is supplied to the loop and is calibrated by the meter which measures the voltage across a sensing resistor to indicate the measured value.
Often the LCD meters are used in areas where ambient light is insufficient for proper viewing or they may be encased in explosion proof or protective housings which limit the ambient light available to the LCD. In such applications independent lighting from a separate power source is necessary in order to illuminate the display.